Conventionally, electric tools having a function of locking an output shaft (spindle) when the motor is controlled to stop as described above are known (for example, Japanese Laid-Open Patent Publication No. 11-37187, hereinafter “Patent Document 1”).
A lock mechanism described in Patent Document 1 has a play angle between an output shaft and an input shaft and includes, on the output shaft side, a locking plate which is urged radially outward (in a locking direction) and a holding plate for restricting the position of the locking plate by a guide groove.
The lock mechanism acts as follows. While the input shaft is being rotated (driven), the holding plate restricts the locking plate at a radially inward position. Therefore, the output shaft is not locked and rotates freely. When the motor stops rotating and thus the input shaft stops rotating, the output shaft is rotated by a play angle because of inertia. Therefore, the locking plate associated with the input shaft moves radially outward through the guide groove of the holding plate. As a result, the output shaft is automatically locked.
As described above, the lock mechanism described in Patent Document 1 instantaneously locks the output shaft using inertia when the motor is stopped. Therefore, the operator does not need to pivot the output shaft by the play angle after the motor is stopped, which significantly improves the operability.
However, the lock mechanism described in Patent Document 1 which uses inertia has the following problem.
When, for example, the motor is stopped after rotating at a low speed, “pivoting because of inertia” is not generated to a sufficient degree. Then, the output shaft is not automatically locked.
In this case, the following occurs. If the operator pivots the output shaft in the same direction as the direction in which the output shaft has been rotated, this means that the output shaft is pivoted in the same direction as the “pivoting because of inertia.” Therefore, the output shaft is locked. By contrast, if the operator pivots the output shaft on the opposite direction, the input shaft is also pivoted in the same direction as the output shaft. Therefore, the relative positions of the locking plate and the input shaft are not changed. As a result, the output shaft is not locked.
When the output shaft is not locked as described above, the lock mechanism does not provide its function. In addition, since the output shaft is not locked, the operator needs to pivot the output shaft in the state of receiving a load of the stoppage of the motor for an extended period of time. This deteriorates the operability.
In order to solve this problem, Patent Document 1 adds a lock operating mechanism using a planet gear set with such a structure, in whichever direction the operator may pivot the output shaft, the relative positions of the locking plate and the input shaft are necessarily changed so as to guarantee that the output shaft is locked.
However, in the case where such a lock operating mechanism is added, a complicated mechanism of the planet gear set is additionally required, which may lower the reliability, durability or the like of the lock mechanism itself. A space for the lock operating mechanism is also required, and the lock mechanism cannot be formed to be compact.